Append -dirty to Git hashcode when working tree is dirty

[joshessman-llnl]

This replaces rev-parse with describe, which was introduced in Git 2.4.12 (May 2017). Do we need to support versions of Git older than that?

[kennyweiss]

@joshessman-llnl wrote:

This replaces rev-parse with describe, which was introduced in Git 2.4.12 (May 2017). Do we need to support versions of Git older than that?

The change here is small enough that it seems like we should probably check the git version and use the appropriate command.